There’s the fact that the experienced internet users are unlikely to install the Comscore kind of software.
Shared computers are a big problem, too: Comscore keeps registration of the characteristics of the user who registered and installed the software. That means the usage patterns of families with shared computers cannot be reported on reliably. However, the fact is that there isn’t really a good way of measuring this kind of thing. Hitwise gets fairly accurate traffic data by buying traffic logs from ISPs, but that can’t show the characteristics of the individual users. However, I tend to trust the Hitwise traffic data more than other sources for general traffic trends. I tend to regard Comscore & Alexa data as “possibly indicative but not definitive, and always requiring further analysis”.
